This invention relates to high voltage fuses, and in particular it relates to high voltage fuses of the current limiting type.
High voltage, current limiting fuses normally comprise a fusible element, or more simply a fuse element, totally enclosed in a tubular casing or cartridge with a pulverulent arc quenching material filling the casing and in contact with the fuse element. Fuses of this type which are intended for higher voltages and large current limiting capacities requires (a) a long fuse element to provide for many small arcs to develop along the length thereof under fault conditions, and (b) a fuse element with a large surface area to provide increased contact with the arc quenching material. In order to have a long fuse element without unduly increasing the length of the casing, the fuse element is usually in the form of a helix. The fuse element is commonly wound on a central supporting core of ceramic or other insulating material to form a helix. In order to have a fuse element with a large surface area, a flat ribbon-like configuration for the fuse element is usually preferred to a wire-like configuration. The ribbon is wound flat on the core. It will be apparent that, for the same spacing between turns of a helix, and the same size of supporting core, a flat ribbon will require a longer casing to contain it than an equivalent wire fuse element and this may compromise the fuse design.
In some applications, for example in a current limiting fuse section of a combination fuse for use in the protection of capacitors in a power system capacitor bank, a fuse element having a length of the order of six feet might be desired. Such a fuse is required to carry high transient currents and to control fault currents. This capability is improved by large surface contact between the fuse element and the surrounding arc quenching material. In the past, it has been difficult to design and produce a current limiting fuse which has a long fuse element, is of the ribbon type for larger surface area, and yet is enclosed in a casing of convenient length.
Accordingly, it is a general object of this invention to provide an improved current limiting fuse which has a helically formed fuse element made from ribbon-like fuse element material wherein the fuse element is capable of being contained in a casing of convenient length.
It is another object of this invention to provide an improved high voltage, current limiting fuse having a helically formed fuse element of ribbon-like fuse material where the ribbon is on edge with respect to the axis of the helix.